1. Field of the Invention
The present invention relates generally to volumetric flow measurement devices, and more specifically relates to a closely coupled, dual turbine volumetric flow measurement device.
2. Description of the Related Art
It is common in the industry to measure the flow of fluids in a conduit by measurement devices that are integral with the conduit. For example, it is common to use orifice plate flow meters, mass turbine flow meters, or volumetric turbine flow meters in line with the conduit or pipeline transporting the fluid.
In recent years, the flow measurement industry has become increasingly concerned with the accuracy of flow measurement, with the predictability of flow meter failure, and with the redundancy or "fail safe" quality of flow measurement. Several attempts have been made by those of skill in the industry to address these issues.
For example, U.S. Pat. No. 3,210,987 to A. C. Ackerman issued on Aug. 24, 1965, and is directed to a mass flow meter. The Ackerman patent discloses a flow meter that employs two turbines. The first turbine is a free running turbine that is operated by the fluid to be measured. The fluid then passes through a flow decoupling device before reaching the second turbine. The second turbine is loaded in such a manner that the power generated by it is a known function of its speed. The flow decoupling device is disclosed to comprise a plurality of high solidity vanes that are mounted in the inlet section to remove swirl from the flow, dampen large scale turbulence, and minimize flow distortions.
U.S. Pat. No. 3,710,622 to Hammond et al. is directed to a viscosity compensated dual rotor turbine flow meter and issued on Jan. 16, 1973. The Hammond patent discloses a dual turbine flow meter that uses an intermediate vane assembly as a spacer between the two turbines and as a fluid flow turbulence reducer. The intermediate vane assembly between the two turbines is disclosed to substantially reduce the interaction that might otherwise exist between the rotor assemblies.
U.S. Pat. No. 4,305,281 to Lee et at. is directed to a self-correcting, self-checking turbine meter and issued on Dec. 15, 1981. The Lee et al. patent discloses a turbine meter that utilizes two turbines. The upstream turbine is disclosed to be the metering rotor and the downstream turbine is disclosed to be a sensing rotor which senses changes in the exit angle of the fluid leaving the metering turbine.
These prior attempts to improve the flow measurement of fluids by utilizing dual turbines has not been entirely successful. For example, prior art dual turbine flow meters often require large physical spacing between the turbines. This results in flow meters that occupy an undesirably long length of conduit or pipeline. Some flow meter installations, such as in Europe, require that the flow meter length be three times the nominal diameter of the conduit or less.
Also, prior art dual turbine flow meters typically are not capable of providing flow measurement if one of the turbines fall. In other words, the turbines of prior art dual turbine flow meters are not independent.
Additionally, prior art dual turbine flow meters are complex and often require substantial post-transduction solution to provide usable flow information.
The present invention is directed to an improved dual turbine flow meter which overcomes or minimizes these and other limitations of prior art devices.